Having contract information associated with a baseline MLS import is not imperative. In many cases, the MLS will represent contracts that are 10+ years old and aren't as important as the purchase data itself which identifies the base license and relevant upgrade paths (be that through the old upgrade models eg. VUP, CUP, PUP etc. or via SA renewals). The effort to associate the contracts may not justify or result in any business benefit - but this is something that the organisation will have determine ie. what is important and relevant.
My recommended approach would be to add the current contract to your system and associate all current products with SA (assuming your organisation has an EA) to that contract. You can then manage and associate this contract (and future contracts) to the relevant entitlements and licenses as they are transacted moving forward.
If it is decided that you want to add contract details post entitlement import and license creation, this can be done by manually associating the contract(s) to the relevant license record(s). If there is significant volume, you would want to look at doing this via a business adapter.