A comment for posterity for anyone getting stuck converting tex to DDS or importing:
For some reason this tool gets stuck if you have many textures for one assets file, so if you want to install something big (for example shared3 in My Winter Car mod has around 110 textures) you will need to divide it into chunks of 30-40 textures max. Then install each chunk separately.
If it gets stuck importing textures you'll need to cancel it (CTRL-C or close the window), then verify game files in Steam because you just corrupted the assets file!
This took me a few tries but eventually worked when I did everything correctly (for My Winter Car I had to divide shared3 into 3 separate installations or it'd get stuck either converting or importing)
Hope this helps people, and pls fix author!
Please, send me your textures with problem: mysummercarbr@gmail.com