Two ways to do this (at least - probably there are others, anyone else have a favorite?).
The most straightforward is just to make an image with the text you want and a transparent background (.png format with alpha level transparency works for sure). Then import that image as a billboard and move, size, rotate it as needed to put the text over the helmet where you want it. Finally, set the billboard’s vehicle to the helmet so the two will move together.
You can sometimes get funny angle effects with the billboard approach. A way to avoid that - although it's more work - is to actually create an alternate texture for the helmet. To do this, make a copy of the .a2c file for the astronaut (don't work with the original - it is very easy to make a mistake

) and decompress it using 7-Zip.
(I think Winrar also works. WinZip usually doesn't get everything but if that's all you have, give it a try first. You‘ll have to change the file type to .zip - another reason to make a copy first. See Wiki for the download sites for 7-Zip and Winrar.)
Now find the texture file you want and edit it to add the text on the helmet. Save the edited image and import it back into Alice as an alternate texture for the astronaut - then just change textures whenever you want the text to appear.
Haven't worked much with the astronaut specifically, but usually you can do the last step either for the whole object or just for parts of the object - in this case the helmet or visor. The advantage of just doing it for a part is that you don't have to worry about messing up other parts of the texture map while you're making the changes you need.
I did take a quick look at the texture file (attached) and it may take a couple of tries to get the text looking right if you go this route. The UV (texture to object surface) mapping looks a bit complex, which can cause image distortion. (I guess more accurately, the UV mapping process
is image distortion.

)
One trick if you are thinking of modifying a texture map is to take a quick look at the texture by making a scene with the object and a billboard (any texture will do for the billboard). Then change the texture on the billboard to that of the object and take a look. Don’t do this to get a texture to edit, though - there will be enough scaling errors that the texture won’t map well to the objects surface.
Hope this helps.