Contribution status for tax deduction?
So if I'm understanding this correctly, you currently have monthly collective totals in Civi and now want to import the detail behind those so you can do reporting at the individual level.
If you have the individual level detail to import, do you still need the monthly entries?  If not, just delete the summary ones and import the details.
If you do want both:  you suggest adding a new contribution status.  There is a lot of logic based around those statuses and I wouldn't mess with that.  But slightly changing that idea you could add a new financial type such as 'imported-monthly-total'.  Then change those monthly totals to this new type so that you can import the detailed data and in the tax reports be sure to filter by financial type to exclude 'imported-monthly-total'.