I am added pak files from baseq2 from my quake 2 folder to quake 2 rtx folder, but how i can play esxpansions?

Copy all files from base Q2 GOG's and place them without overwriting to Q2RTX installation (GOG's 1.5 or Github 1.6 versions).

Start it as any addons for base Q2 or any sourceports mods (with new links and +game-<...> argument).

Or you even can still use Q2Launcher.exe! Just need some tweaks to make QLauncher work properly. Edit Q2Launcher.ini file, rename it to Q2RTXLauncher.ini, edit link file copied from base game folder to new one - correct path change ini file to new one, change game id to new one. I guess Q2RTX GOG's game id shudl be ok... or you can put htere any gameid. In that case you need add GOG.com\Games\*newid*\ to windows REGEDIT.
Look here
http://www.gog.com/forum/quake_series/using_the_gog_quake_ii_launcher_with_two_separate_installs

Google also say you need additional fork-mod patch for Q2RTX, i guess that not true from one of first Q2RTX updates. It is not he case since that and for current 1.5\1.6 versions.

*in case if you first install RTX as 'demo', you need overwrite baseq2 pack files in RTX baseq2 folder.