You can convert just about anything to anything! Now, is it practical? Probably!
You have a 10-15-20 meter yagi with aluminum parts. All you need to do is adjust everything to the frequencies you wish to cover.
True, you can convert just about anything to anything. But I'm not so sure it's as simple as just adjusting things. The TH-5 is a trapped tribander, and the traps will be of little or no use for 12M & 17M. So the 10M and 15M traps would have to be replaced and then all of the dimensions scaled for the two new bands. To use the 10M element you'd have to lengthen it for 12M, which may or may not be easy. If you're up for all that, then go for it.