Can I import my old iTunes library into Apple Music?

Yes. Apple Music shares the matching component of iTunes Match. Locally stored songs in your iTunes library will be matched or uploaded to join your iCloud Music Library. If you've turned on iCloud Music Library/Sync Library in the current library then it should have happened already. If you've created a separate library for Apple Music you can import any of your existing files as you choose.

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See How to use account recovery when you can’t reset your Apple ID password - Apple Support. It should be possible to regain access to the account. iCloud is an umbrella term Apple use for a number of features associated with an Apple ID, not a separate account. The Apple Music subscription service works with an Apple ID. While your locally ripped music can be added to to the cloud library, Apple Music will see tracks purchased with a different Apple ID as ineligible. Ideally you would regain control of your original account, then change the email address associated with it. Alternatively you can convert old purchases to new AAC copies that don't contain a link to your old account, and those copies can be assimilated into your cloud library.

It is worth noting that some people seem to have issues when joining large libraries to Apple Music, though this may be a matter of how well organized the libraries are in terms of accurate metadata, duplicates, etc. Before starting you ought to have a complete backup of your library that you can access independently if needed. It is also possible to use a separate library for Apple Music and your main library. Then you can gradually introduce your own content into the cloud library rather than doing it all in one go.

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Perhaps this user tip will help:

Move your iTunes library to a new computer - Apple Community

But if you don't know the Apple ID or password for that library, that may be a problem.