This is certainly doable however, I don’t know that I would recommend it. Once you have all of this non-posted data in Method, you’ll need to be very careful when making changes to keep this data separate.
When importing your customers, I would not recommend importing them as leads. Some of these customers will already exist in Method. If you import them as leads, this will conflict with the data that is already in Method.
You’ll also need to be very thorough with your cleanup. If your key fields don’t match exactly, upon import you’ll end up with duplicate records.
When dealing with transactions that you don’t want to post to QuickBooks (QB), you’ll need to use the IsWaitingForSyncApproval field. This is the only field that will prevent the transaction from posting to QB.
One thing I would definitely recommend is that you have an up-to-date QB backup before doing this. Should anything go wrong, for a consulting fee, we can roll back your Method account to the previous backup. Backups are done once daily. It’s not ideal, but at least you will have a fallback should everything go wrong. I don’t think you’ll need this, but better safe than sorry.
If you wanted to test this on a “sandbox” Method account, you can sign up for a trial on www.method.me. Just sync the data from your sandbox QB file to this account and you should be good to go.
I hope this helps and please let me know if you have any other questions. I’d also definitely be interested in what your end result is and if you ran into any issues during the process.