Marshal Class for other file formats?

I've been working on a program that does image modification lately, and I've been using the marshal class for it's fast mem usage. My question is, can I load other files into mem and do the same thing? I have a file thats basically a 16BPP bitmap that just has a different header structure than a BMP and if i can get into memory, then i can VERY easily reconstruct the header to a BMP header. I'm currently doing this using a file "OPEN" command, and outputting the image into a new file. Takes about 2-4 seconds per file.