If you are sure that the xap file is not already MDIL compiled, then you can use the powerShell scripts as outlined in the below article to sign it with Enterprise distrubtion. You have to choose the correct script based the version of your app (8.0 or 8.1)
https://msdn.microsoft.com/en-us/library/windows/apps/dn168929(v=vs.105).aspx
A quick way to test after signing is , install the AETX token first and then the signed app, using email on phone.
