tkapc,
I don't know of anyway to change an opentype to a truetype, you'd probably have to ask the folks who make fonts, (not sure where you'd find them) but they may know. I'm not sure this is something you necessarily have to do - what you may want to do is check your windows fonts, control panel - fonts, and then see which font types are listed as opentype, then check AutoCAD LT to see if it is recognizing any truetype fonts. As far as a setting for this I'm afraid I'm not familiar enough with LT or 2005 to be able to answer but I doubt it.
I have a feeling your font file is the problem, not AutoCAD. Have you tried using the font in something like Word, Excel or any other program just to make sure it works?
If you can't use it in any other program, you won't be able to use it in AutoCAD.
HTH
Todd