Left panel: In the baseline,
Right panel: Results of a search query.
Drag & drop from a panel to another.

Original request:

My client's way of working is to frequently add/remove requirements to a baseline, then refreeze. This works much better for my clients than searching for requirements, then baselining.
They feel more at home when starting from a "known" baseline which they manipulate...they use baselines as a marker or shortcut query for a given set of requirements.

Thus, I suggest to make adding/removing easier:
How about a drag+drop dialogue, with 2 columns: Left "in the baseline", right "requirements matching your filter criteria", then I can drag/drop into the baseline?