Unfortunately, I don't have many of my sources around at the moment (just moved and most of my books are either still in boxes or in storage), but I'll try and give the best answer I can. Anything dealing with the legal structure of the Holy Roman Empire is very complicated, given the unwritten nature of the Imperial Constitution, so this is, of course, a very general answer, but hopefully it will suffice.

There was a definite degree of push and pull within the HRE over who had the right to create new nobility. For the Emperor, espescially Ferdinand II, the right to distribute land and create new noble titles was an inherent power of the Emperor, which he used quite often. Indeed this power of the Emperor underwrit much of the Imperialist war effort throughout the Thirty Years War as, while the Emperor was unable to pay the wages of his armies directly, he could promise them lands and titles in exchange for forgiveness of arrears. Similarly, confiscation and redistribution of land was an extensive tool used by Emperor Ferdinand II in destroying the power base of rebels, while rewarding loyalists, helping to ensure that the local power base in the Austrian Habsburg Hereditary Lands remained loyal.

Of course, like almost everything within the Holy Roman Empire, there was a degree of pushback. Even among the Princes who supported the Emperor, like the Duke of Bavaria, there was significant worry about allowing a precedent to be set that future Emperor. The general opinion held by the *Reichstag* was that, while the Emperor could create new titles, he could only do so in consultation with the *Reichstag*. This was what was ultimately settled on at the end of the war in the Peace of Westphalia: that the Emperor could create new noble titles in consultation with the Imperial estates.

Outside of the legality of course, there were social issues present when it came to creating titles, espescially high ranking title like dukes. Much like how the Emperor was supposed to be Imperial inherently in order to be considered for the title, there was also a deal of social unrest when new nobility seemed to be rising above their station. Alas, this is something I'm not overly familiar with, so I can't do much more than acknowledge its existence as a factor.

So, to bring things back around to your actual question: if by *Fürst* you mean any old Imperial Prince with Imperial immediacy, then no, they would not be able to create a ducal title at a whim. That power would almost certainly lie solely with the Emperor, potentially in consultation with the Electors or the Diet as a whole.

Hopefully this helps to answer your question. Please feel free to ask any follow ups.