Yes it is! But you need to create a new keyset to do this

You can create a keyset from an existing nsec or a new one. A keyset is a set of FROSTR "shares" (basically like keys in a multisig) and you need a threshold of these shares to sign a valid note (you choose the threshold)

These shares in the set are inherently interconnected so if you have a 2/3 and you want to make it a 3/5 you need to DESTROY the old 2/3 keyset (or at least the threshold of shares in that keysey) and then CREATE a new 3/5 keyset (with the same nsec)

Demo is here: https://x.com/bitcoinplebdev/status/1905056787771842665