It is actually possible to use with UserPro, as that plugin stores the user fields as user meta – which is possible to search for.
You can find the field names under the UserPro fields panel: https://i.imgur.com/PVmAcdH.png
..and these can be entered in Ajax Search Pro under the General Options -> User search panel: https://i.imgur.com/5Y8Y4EP.png
Any text based field will work.
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