As far as I know - yes and no.
The MySQL ALTER TABLE command has a DISABLE KEYS clause, but only for nonunique indexes. So, this is irrelevant for primary keys.
The problem with that is that the constraint is supposed to be considered its own entity, entirely separate from the table. of course it only exists as long as the table exists, but its status shouldn't be controlled by the table in that way.
So yes, you can disable a key by altering a table but you can't alter a key's status by itself.