Approach the Quilliam Foundation in the UK. This is a tough ask and you'll need to carry a great deal of trust and credibility with you. I suspect that won't come overnight.
Thank you Dominic, I did my doktoral research on converts so I know how difficult such research is. Still, I'm asking for secondary sources not raw data.
Terrorist's Creed - Roger Griffin. Interesting read. The sort of book that starts making a great deal of sense after the third or fourth read! At a more granular level - helps frame the Griffin book - read The Sacred Canopy - Peter Berger. Fascinating book pub in 1964 and still completely relevant concept. Was far ahead of its time IMHO.
Monika, Like Islomjon, I am concerned whether your question links converts (in general) to Islam with ISIS 'converts.' ISIS and Islam barely belong in the same breath, since ISIS violates and perverts the most basic tenets, principles, and practices of Islam. Even those Muslims who join ISIS are violating their own "deen" or way of life as laid out in the Qur'an and the Hadith of the Prophet. I am an American convert to Islam and can tell you that I, like millions of other Muslims, am appalled at what ISIS has been doing in the name of "Islam." I studied the original texts for several years and did not convert for psychological reasons at all--but for the religion itself. If one studies Islam in depth, and does not "cherry pick" elements to emphasize, dislike, or support in the extreme, then, it becomes easier to understand what the sacred texts teach, advise, and suggest. That is the greatest difference between converts to Islam and converts to ISIS. ISIS "cherry picks" phrases here and there with total disregard for the context in which they were taught; not one word from the Hadith supports any of the actions of the so-called "caliph" Al-Baghdadi.
I would love to provide a more "on point" response, if you clarify a bit more what you are looking for in the realm of converts. The subject is quite broad and there are a multitude of responses.