Yes it's possible to combine more pollers in the transformation formula. Just make sure that pollers you'll use for transform are assigned on the same node(s) and have the same polling interval.
To your original qustion:
Universal Device Poller Status resource doesn't allow any filtering. Easiest thing you can do is change the group name for your UnDPs. You'll still see all the values but organized under different group. For example 'APC original' and 'APC transformed'. Group cold be changed in the UnDP application when editing UnDP.
Another option is to use Custom Query resource and select only the UnDPs you're interested in (transformations in this case). Just place 'Custom Query' resource on some node details view. Edit this resource and paste following code as a Custom SWQL Query:
SELECT c.UniqueName AS [Poller Name], s.Status AS [Current Status], ToLocal(s.DateTime) AS [Last Polled]
FROM Orion.NPM.CustomPollerAssignment a
JOIN Orion.NPM.CustomPollers c ON a.CustomPollerID = c.CustomPollerID
JOIN Orion.NPM.CustomPollerStatus s ON a.CustomPollerAssignmentID = s.CustomPollerAssignmentID
WHERE c.PollerType = 'F' AND a.NodeID = ${NodeID}