I have seen here the NT being used to explain or elaborate the OT verses , its fair seeming to allow users to use the Quran which Muslims consider the FT (Final Testament) to explain and elaborate the Biblical verses.
No, but the restriction isn't in this site's policy so much as the nature of the Qur'an and its experts.
The scope of this site is determined less by a specific definition of "book" than it is by a certain field of knowledge. That field of knowledge is the sort needed to read and interpret the Greek and Hebrew texts as well as background in, textual criticism, history and other things related and possibly necessary. This site is centered around the people that are interested in those fields.
Notably, people with expertise in the Qur'an typically have no expertise in the above fields. There is remarkably little cross over between these fields. The few exceptions tend to be in the same vein as somebody who is an Oracle certified database administrator and holds an EMT license.
There are of course experts in the Islamic world with an interest in the Bible, but the interest is of an entirely different sort than that of the experts we are focused on. This site requires that all exegesis answers start from the text and work up, showing their interpretive process. By definition the Qur'an is not particularly useful for this because it doesn't claim to explain Biblical verses. It does give alternate versions of some stories and teachings, but that is not the same as interpreting them.
Islamic experts approach the Biblical text from a specific doctrinal perspective that is not within the general scope of this site. This site isn't about what should be believed, it is about the texts. Islam does not believe in the texts themselves. It claims to believe in some theoretical version of some of the texts that are no longer extant to be studied, but it does not believe in the Bible as we have it to study today. Hence it is, by definition, not really relevant in interpreting the texts.