The PSR-I and PSR-E models don't support the saving and loading of .RGT registration files, so you aren't going to be able to use the file on your PSR-I455.
The PSR-I455 does have registrations, and you can save and load the registrations as part of a .USR file. However, .USR files contain some other things besides the registrations-- namely, the user songs and user styles.
I know of no existing software which can convert .RGT files from some other keyboard into .USR files for the PSR-I455.
What you'll need to do is use a registration editor computer program to inspect the registration entries contained in the .RGT file, then make a list of the various settings used in each registration and the nearest equivalent setting (if one is available) for the PSR-I455. After you've made your list, you can manually set up the registrations on your PSR-I455. When you're done, you can save your PSR-I455's memory to a .USR file for safe-keeping.