I can see the culture of Westfriesland being Frisian, but a claim? I don't think the Brunonen Counts of Friesland ever had much interest in seizing the western Frisian lands of the Holland family. And for now the game has Frisia being alternately part of Sticht, Gelre or Holland. One of them were already lords of Westfriesland, the bishops already had a claim for seperate reasons and failed, and the last couldn't even get the eastern part of Friesland under control. Friesland would need to have a reigning noble family of its own in the game that can have claims. And since Friesland was more similar to a republic than a county, we'd probably have to wait for Paradox to make some expansion that allows you to play one, see if Friesland gets changed to reflect that, and how such an add-on would handle claims by a republic.
I hate to rub it in your face, but you'll have to be content with the Hollanders remaking the Kingdom of Friesland. They have Frisian ancestry though, so that makes it almost acceptable.