If your Installshield MSI is a standard MSI and not an MSI with customizable dialog boxes (commonly called MSI Installscript), it should be possible to create an update using WIX. This does not mean that it will be easy. You will need to map all the GUID components of the component and it is preferable to keep the directory table structure the same, among other things. It can be a serious problem.
If you still want to try, I would decompile Installshield MSI with dark.exe, excluding unnecessary sections such as Installshield GUI dialogs (if you don't have custom logic in your dialogs), and generally try to simplify things as much as possible. Wix has ways to insert the necessary MSI “default elements” (for example, a GUI) if you omit the code for it in the WIX source file and include the default values.
In general, I would not recommend this if you had no prior experience with WIX and Installshield. I have one, and it seems to me that it will take me enough time to complete it successfully, but it all depends on the complexity of the installation itself.