I never have done this so I'm just assuming some points based on experience. maybe Gigazelle can add some points if I miss them...
first: don't do it 
I know, sound strange. but based on my experience with both tools, neither naming nor counting metrics ever match up. even if its the same idea, the logic is not the same ... I personally wouldn't go that way...
more reasons later on...
historic data
basically there is a way to import data to adobe analytics. as far as I know you can export google analytics data, modify it and upload to adobe analytics. but if you do this, you would be billed by each row you import (per server call billing).... at least that is the normal way. ask your adobe contact or customer care about the import possiblities and format
but: new visitors
however, if you decide to import the data you would break "visitors" (users). that means adobe has its own visitorID and they wont fit. that means you would have a new visitor for each person who comes to your website for the first time after changing tool. guess this will need some explanation to the business
workaround: summary data
I don't know what exactly you want to achieve. but you could upload some vasic summary data (eg. daily summary of visitors for each site section). as far as I know you wont get billed for this... maybe this is a way to get at least some high level metrics into adobe analaytics and not loosing the history completely...
further resources:
- How to Get Data In and Out of SiteCatalyst - Part I | Adobe Blog
- Data Sources: Experience Cloud Help