Custom Membership Start / Stop Dates

Hello -
We are intending to replace an existing memebership site with the Membership Plugin from WPMUDEV.

We'll need to, by the looks of it, manually set up our current paid members into the new system, however there is no option to customize their start and stop dates of their membership.

This is extremely important for our business model.
I've looked through the database as well, and it seems like the date field is a random string, not an actual date that can be modified.

Would it be possible to get some guidance on how we can accomplish this?