FOSS gamedev and creative worlds

Before converting a Minecraft Texture Pack to Minetest do I have to ask the maker of the original? or can I convert it and post the original topic from the minecraft forums unless that's considered advertising?

It depends on the original author's license. If he or she doesn't specifically state their license, you need to ask them. Otherwise, they'll probably say it's public domain, or WTFPL, or Creative Commons, or whatever, in which case you only need to properly credit that person when you release (no permission necessary, generally).

So I have to ask if they don't say the license?

It depends on 2 actions.

1- The texture pack is for you and you alone.
2- You plan to release it to the public. Us or anyone else to download and use.

If you what you want is 1, then do it now.
If you plan to follow option 2, then check if its a public texture pack(I think Dokucraft is as an example) if its not send the author a message and ask him if you can do that.

you must ask the author of a texture pack its only polite to do so and could save trouble if the author does not wish it and finds out that youve done it behind his or her back (not a likely scenario)