OK,I was messing with GrafX2 and it seemed that if I choose the file format first, lbm in the format box by using the up or down arrows there, it is easier to then backspace the two dots in the name box and then type the new name, "10001" and save as a new lbm file. This way you do not have to overwrite.
Flip, I am not sure how to really solve the trouble you are having otherwise with GrafX2 so I am going to write a quick guide for using a lbm and tga saved with a more modern program like paintshop, and then using seqgrab to save the tga as a bif.