I created .txd from txdworkshop but I have to do to create .dff
I'm assuming you only want the texture as TextDraw and not the actual model placed ingame?
If so, you don't need a model that actually uses the Texture. You can use any (valid) dff you can think of (preferably an empty one as it's as small as possible).
It also doesn't have to include a SAMP Collision as it will never actually be loaded.
If you have IMG Tool you can check SAMP.img for "NoModelFile.dff" and extract it. It's an extremely small model you could use to load your TXD.
Otherwise learn a bit about Blender, you could literally export the default model that's loaded on start-up as DFF using the DFF exporter.