NoTeaForMe - I've had a quick look at your other threads, and I'm gathering that you are suffering a lot of pain and nipple trauma. Some of your posts could have been written by me a few months ago (and probably were if you were to search under my nickname!) Given what you're going through I can totally understand why you are talking about expressing rather than feeding directly.
Firstly, I want to make a suggestion to you which is to try nipple shields, if you haven't done so already. There are all sorts of caveats about potentially reducing supply, teaching baby bad latching habits etc. etc., but they are definitely definitely worth a try before introducing formula. They take the edge of the pain and allow your nipples to heal. They have saved me twice.
I have some experience of the type of mixed feeding you are suggesting. My DS (now 4) had tongue tie which went undiagnosed, and feeding him was total agony. I'm sorry to say I did not last as long as you have before I had to introduce a bottle. I did a mixture of EBM and formula, with some direct feeding. The only way I could feed directly was with nipple shields.
I can tell you that it did work - DS continued getting some breastmilk for 4.5 months - BUT it really is the worst of both worlds. I have a very very detailed feeding diary in which I recorded how much EBM, how much formula and how long on the breast - some feeds comprised all three. Later on I separated the feeds out so that some were breast and some were formula. I think the latter method was probably better.
With my DD (20 weeks) I have had similar problems and was nearly forced into the same position. I was not going to go down the mixed feeding route again and nipple shields prevented me from having to do this. I did give formula overnight though for 12 weeks - not generally recommended because of the impact on supply but for me it was that or give up. We're now breastfeeding exclusively again and DD is nearly 21 weeks.
WRT to mixed feeding specifically, if you want to maintain your supply then do make sure that as your DD's appetite increases you try to match that with more EBM/extra breastfeeds if you can manage it, and not more formula.
Any questions feel free to ask. It's hard to remember now, but specific questions might jog my memory!