Brian White created a Babble discussion in 2007 on combining the single transferrable vote system and MMP from an STV-preferred perspective:
STV and MMP proponents were at each others throats in BC before during and after the referendum.(and it did nobody any good)
Perhaps a proposal like this might get most of them onside?
In his proposal, the added members would amount to only 25-30% of the total number of seats.
Coming from an MMP-preferred perspective, I would propose that no more than half of the total number of seats should be based on districts or geographical constituencies. That lower limit lowers the risk of overhang. That risk of overhang would be reduced further if all district / geographical constituency seats were to be elected on the basis of preferential voting / ranked voting, and if all but the most sparsely populated geographic constituencies were to be consolidated into multi-member ones.
I believe this is where such STV component under MMP would fulfill an invaluable "fair vote" role.