Please wait for some community response, too, but here's my answer:
I would say it is on-topic.
You're essentially asking about creating your own handmade crafted good. Similar examples would be scented candle and soap making questions.
Hand-made, scented goods are a very popular, and lucrative, craft and I see no reason to rule them out.
Some fuzzy litmus tests I apply here:
- Is the process done "by hand"?
- Is the product tangible?
- Is the product something that's used in the creation of another handmade process?
You'd (probably) be doing this "by hand" (not through some automated process).
The end result is tangible: myrrh resin with a specific scent. Although the scent isn't tangible, the resin is.
The product will be used in creating incense, and could possibly be used to create other scented goods.
Those questions are not hard-and-fast rules, by any means, but helped me categorize this. I see this as related to asking how to mix your own pigment or dye.