Create a new database and import into that. I don't think there is a size limit on import just on opening base PGN files and the clipbase.
As to merging files, they are just text so you can use command line utilities to merge/concatenate them together. Just do a google search for how to do that. I have a folder with a batch file I to do it.
I have purchased the Opening master database and am using the Scid vs PC software. Scid vs PC seems to limit the size of the databases I can open.
1. how can I merge all of the Opening master PGN files?
2. How can I open big PGN files (more than 10 million games)?