Please use the edit button instead of double posting.
You can try decompiling the Goldsource BSP but you will have trouble modifying and compiling it because decompiled Goldsource maps are terrible. On the other hand decompiled Source BSP files are much cleaner and can be modified easily. The steps I listed will let you convert the decompiled Source map to RMF so you can use in whatever editor you like.
You might need to do a search & replace on the VMF in notepad before converting to RMF to convert Source texture names to Goldsource texture names, but that won't be difficult.