Since Caste is a social phenomenon and not a legal one, therefore no laws exist for determination of castes for person who convert to Hinduism from other religions.
But there are many ways to "assign" castes to such persons by way of :-
1. If they remember what caste his/her ancestors belonged to before they embraced their current religion. Remember the ancestors of 95% of such persons were Hindus.
2. If they dont remember the caste, the caste can be assigned on the basis of profession their ancestors have been doing.
3. If that is not possible, the priest converting a person to Hinduism may ask some Hindu to "adopt" that person so as to give him a surname only. This adoption is not a legal adoption but just a verbal vow before the priest. The caste of the adopter Hindu becomes tha caste of the adoptee.
4. Or any other way the Priest may decide.