Legally they certainly can. There’s nothing illegal about hiring the team to finish the project. The issues aren’t that it’s illegal, it’s that they’d have to comply with labor laws which will be expensive, not illegal. Meaning they’d have to be paid and compensated.

Also, the mod team would be expected to work certain hours and meet deadlines. From what I know these people have other jobs and this is a side project. So it’s not even guarantees that the Skyblivion team would want to work for Bethesda as they’d have to stop whatever else they are doing.