You could take a backup of those files before you add RCT1 to OpenRCT2. In general I think you should be safe as OpenRCT2 stores the 'victories' in highscores.dat file in it´s own folder and the same goes for the saves. So it should not delete your scores in RCT1's data folder. But the most important thing is to take a backup of the RCT1 folder with your saves and scores if you want to be 100% safe.