First, copy your font to your system font directory. If you don't get it, go to Control panel->Font, then Click file->install.. Select drive and folder where your font is(do not forget to click..). It will install your font.
Next, run RMXP, make or open a project, then press F11 to open script editor. Go to 'main', insert this line:
Font.default_name = ['yourfont1','yourfont2']
#if 'yourfont1' doesn't exist or certain characters doesn't exist in 'yourfont1', the project will use 'yourfont2'
You can see font name in Control panel->Font(or you can use any font manager program).