You can accomplish this in Nero by using the Start Multisession option on the Multisession tab when you first burn the DVD+RW or DVD-RW.
The next time you want to add something to it, you select the Continue Multisession option and then you can perform a Refresh Compilation from the File menu.
If you delete something from the disc when adding a later session, that space will not be reclaimed on the disc. If you really want to reclaim space, you need to erase the disc and start a new (multisession) project.
It depends on whether the disc was originally burned as a multisession disc.
InCD can work with or without Mt. Rainier support.