diff --git a/crates/graphql/src/mutations/user.rs b/crates/graphql/src/mutations/user.rs
index b469cad4..c38037c8 100644
--- a/crates/graphql/src/mutations/user.rs
+++ b/crates/graphql/src/mutations/user.rs
@@ -18,7 +18,7 @@ use mas_storage::{
     job::{DeactivateUserJob, JobRepositoryExt, ProvisionUserJob},
     user::UserRepository,
 };
-use tracing::info;
+use tracing::{info, warn};
 
 use crate::{
     model::{NodeType, User},
@@ -36,6 +36,13 @@ pub struct UserMutations {
 struct AddUserInput {
     /// The username of the user to add.
     username: String,
+
+    /// Skip checking with the homeserver whether the username is valid.
+    ///
+    /// Use this with caution! The main reason to use this, is when a user used
+    /// by an application service needs to exist in MAS to craft special
+    /// tokens (like with admin access) for them
+    skip_homeserver_check: Option<bool>,
 }
 
 /// The status of the `addUser` mutation.
@@ -47,6 +54,9 @@ enum AddUserStatus {
     /// The user already exists.
     Exists,
 
+    /// The username is reserved.
+    Reserved,
+
     /// The username is invalid.
     Invalid,
 }
@@ -56,6 +66,7 @@ enum AddUserStatus {
 enum AddUserPayload {
     Added(mas_data_model::User),
     Exists(mas_data_model::User),
+    Reserved,
     Invalid,
 }
 
@@ -66,6 +77,7 @@ impl AddUserPayload {
         match self {
             Self::Added(_) => AddUserStatus::Added,
             Self::Exists(_) => AddUserStatus::Exists,
+            Self::Reserved => AddUserStatus::Reserved,
             Self::Invalid => AddUserStatus::Invalid,
         }
     }
@@ -74,7 +86,7 @@ impl AddUserPayload {
     async fn user(&self) -> Option<User> {
         match self {
             Self::Added(user) | Self::Exists(user) => Some(User(user.clone())),
-            Self::Invalid => None,
+            Self::Invalid | Self::Reserved => None,
         }
     }
 }
@@ -245,6 +257,21 @@ impl UserMutations {
             return Ok(AddUserPayload::Invalid);
         }
 
+        // Ask the homeserver if the username is available
+        let homeserver_available = state
+            .homeserver_connection()
+            .is_localpart_available(&input.username)
+            .await?;
+
+        if !homeserver_available {
+            if !input.skip_homeserver_check.unwrap_or(false) {
+                return Ok(AddUserPayload::Reserved);
+            }
+
+            // If we skipped the check, we still want to shout about it
+            warn!("Skipped homeserver check for username {}", input.username);
+        }
+
         let user = repo.user().add(&mut rng, &clock, input.username).await?;
 
         repo.job()
diff --git a/frontend/schema.graphql b/frontend/schema.graphql
index 54de662b..46a8de6e 100644
--- a/frontend/schema.graphql
+++ b/frontend/schema.graphql
@@ -72,6 +72,14 @@ input AddUserInput {
   The username of the user to add.
   """
   username: String!
+  """
+  Skip checking with the homeserver whether the username is valid.
+
+  Use this with caution! The main reason to use this, is when a user used
+  by an application service needs to exist in MAS to craft special
+  tokens (like with admin access) for them
+  """
+  skipHomeserverCheck: Boolean
 }
 
 """
@@ -101,6 +109,10 @@ enum AddUserStatus {
   """
   EXISTS
   """
+  The username is reserved.
+  """
+  RESERVED
+  """
   The username is invalid.
   """
   INVALID
diff --git a/frontend/src/gql/graphql.ts b/frontend/src/gql/graphql.ts
index 8fde3d9c..f9a1421a 100644
--- a/frontend/src/gql/graphql.ts
+++ b/frontend/src/gql/graphql.ts
@@ -76,6 +76,14 @@ export enum AddEmailStatus {
 
 /** The input for the `addUser` mutation. */
 export type AddUserInput = {
+  /**
+   * Skip checking with the homeserver whether the username is valid.
+   *
+   * Use this with caution! The main reason to use this, is when a user used
+   * by an application service needs to exist in MAS to craft special
+   * tokens (like with admin access) for them
+   */
+  skipHomeserverCheck?: InputMaybe<Scalars["Boolean"]["input"]>;
   /** The username of the user to add. */
   username: Scalars["String"]["input"];
 };
@@ -97,6 +105,8 @@ export enum AddUserStatus {
   Exists = "EXISTS",
   /** The username is invalid. */
   Invalid = "INVALID",
+  /** The username is reserved. */
+  Reserved = "RESERVED",
 }
 
 /** The input for the `allowUserCrossSigningReset` mutation. */