Some FBX files contains takes. If they do, Houdini creates takes upon import. Both these approaches seemed to work well for me using H19.
Open Take List and left-button drag some take onto Main. A pop menu allows to pick "Merge take (with override)". Simply "Merge take" is not enough as both Main and the unwanted take has values for the same channels.
In a Textport window: takemerge -f Main <name-of-unwanted take> Again it's the -f that does the trick of forcing the merge even though it means overriding Mains animation channels.