HikaMarket Mutlivendor is a different extension than HikaShop. So if you install HikaMarket, you won't loose any modifications to did to HikaShop files.
However, I strongly recommend you to not modify directly the files but use plugins instead.
There are several ways to change the behavior of HikaShop and we published a developer documentation to help developers creating plugins for HikaShop. You will find it here:
www.hikashop.com/en/support/documentatio...r-documentation.html
www.hikashop.com/en/support/documentatio...ize-the-display.html
That should normally reduce to 0 the need for modifications directly in core files of HikaShop.
Note that HikaMarket also has a developer documentation so that you can also modify its behavior without loosing your changes when you update it.