A blank pattern is a number you haven't yet used in the sequence. If you repeat a number, it's not a copy of a previous pattern, it IS the pattern.
SEQ adds the next (
sequential) unused pattern in the sequence. CLN clones the current pattern to the next unused one. INS just inserts another instance of the current pattern in the sequnce. The + and - increment and decrement the pattern number respectively.
To add a blank pattern, simply use a number you haven't used before.
Sure MilkyTracker can load game music, if it was made in a tracker, which is fairly marginal these days. Actually, it always was on the PC where MIDI and Adlib music were more common. There is no such folder or codec support, MilkyTracker can import over 30 tracker formats as is but very few games use them. Exceptions include older Epic Megagames releases like One Must Fall 2097, Jazz Jackrabbit and Epic Pinball which use the .PSM format (converted .MTM), Disney's Aladdin that uses .AMF (converted .MOD) and Unreal and Deus Ex which use .UMX (converted from various standard formats).
It is taking a lot longer than originally planned to support Un4seen Developments' .MO3 format which is MOD/XM/IT/S3M with MP3/Ogg Vorbis/Lossless sample compression. This format is used by some indie and casual game developers nowadays. For now, you need to
unmo3 the files first.
Of course there are also games which use common module formats straight up. But you can't expect MilkyTracker (or any other software) to universally convert any game music to tracker format. Streamed audio has been ruling the market about since CD-ROM games came out.