If you want your users to add their printers in a XenApp-Session by their own you can publish the „search printers“ dialog by doing the following steps
1. Open the dialog locally on a XenApp-Server.
2. May add some custom filters like your Printservers or site location
3. Save the search result as an .qds for example in your netlogon.
4. Publish your application with the following command:
"%windir%\system32\rundll32.exe" with parameter: "%windir%\system32\dsquery.dll,OpenSavedDsQuery \\PATH_TO_YOUR_QDS\nameofyourqds.qds"