Can Iranian users use their services to convert digital currency?
the rial is listed under national currency in bisq. there are no offers, so you will need to create one for other users take it. if you can send rial over internet banking now to other people, you should be able to use bisq. may need to promote the app to other people from your country, there has been no IRR trades in the past.
Btc<->Altcoin should work without any issues, if you want to go to fiat it’s indeed dependent on other people making offers in IRR (or using OKPAY, Western Union, …). Spread the word in Iran 
Also not sure how well Tor works in Iran, but custom Tor bridges were recently added to Bisq, so a report from Iran that at least the app is working and offers are seen would be helpful!