If using 2011, you would be able to import it and create new forms to look like the old one, create security roles that contain the relevant entities and related the forms to the relevant security roles. And lastly modify the sitemap and set priviliges to
keep irrelevant areas from the users.
The MS donation management solution from marketplace, will though rename some of the default entities and fields, and that WILL effect the forms for the standard CRM solution.
Doing the above customizations is not a huge job.. but for a non-CRM consultant that has no experience with the MS donation management solution.. it will take 50-100 hours to run through the system and set up forms, views, workflows, sitemap etc etc etc.
An experience CRM consultant with Donations management experience would maybe be able to do it in 3-4 days.
But for what reason do you want to run both CRM systems in the same organization? Will there be an overlap of contacts/accounts that is important to keep updated in both systems?
Otherwise you should consider just making another CRM organization (or create a new online) and run the two seperately.
/Rune
CRM consultant - ColumbusIT Denmark