Obviously, anything can be done for the right price. There aren't any digital phones that are compatible with the factory hf kit in the NSX. I believe the OEM NSX phone was made by Motorola so the only part you may be able to use is the speaker. Given that it's 10 years old, you may want to replace the whole thing if you are going to get a new system.
Of all the hf kits I have seen thus far, I think the Nokia one is the nicest. Over 99% of phones today are handheld versions. Most car kits offer HF operation only. On the Nokia, you can get a privacy handset that allows you to keep the phone in the cradle, pick up the handset and talk like a normal phone. It sounds great. I don't have one installed in the NSX but I have it in my other cars. Only problem with it is this particular HF kit only works with 6300 series and 6100 series, which aren't as small as some other models.
I have seen pics of the Motorola HF kit that now has a privacy handset. Never used it.
Installing a new phone system isn't much different than installing a radar. I don't know the first thing about installing but the point is..if you find a HF kit that you like, you can get a skilled installer to install any way you want it. For example, I have seen people instal the Nokia kit in the armrest/storage compartment (non-NSX) and install the privacy handset next to the passenger's left leg. That way, it doesn't take up a lot of leg space etc. But...you can't see the caller id on the phone it rings. I've also seen small phones (w/ small HF kits) installed in the dash.
2 cents.