Sacramentally, a bishop has the power to ordain any baptised man as a priest, however, that does not mean he has the right or prerogative to do so.
Within a monastery, it is not the bishop who determines who is to be ordained as a priest. It is the superior of that monastery who chooses which monks are suitable to be priests and should be ordained. These candidates for priesthood then undergo priestly formation and eventually ordination.
Furthermore, faculties within a monastery are given by the superior. If a bishop becomes a monk and is subject to the superior, then whether he may ordain or even celebrate mass is dependent on whether his superior has granted him the faculties to do so. Of course, if the bishop is the superior himself, then that is an entirely different matter.
Additionally, out of custom and respect to the local ordinary, these candidates are usually ordained by the reigning bishop of the diocese in which the monastery is based. The superior could conceivably call in any bishop in good standing with the Church, but this could be seen as a slight to the authority of the local bishop, and is generally avoided where possible. In any case, the decision rests with the monastery superior as to who is invited to ordain his monks. Additionally, if the local bishop is invited to ordain the monks, he could in turn delegate this duty to another bishop, including retired or auxiliary bishops under his charge.
As such, I would conceive that under most circumstances, a bishop emeritus (whether a monk or not) would need the monastery superior and local bishop’s permission before he may ordain monks to a priesthood.
I must ask what are the circumstances you are thinking of. If there is an example, please name it. I find it highly irregular and bizarre to speaking of a retired bishop becoming a monk. The two are entirely different callings, and while there have been religious that have become bishops, the converse is very rare. Additionally, it must be noted that religious who are ordained bishops are released from obedience to the superiors of their orders (though not to their vows), but I am not sure how it may work out when a retired bishop becomes a religious. Does he become subject to the superior?
If you are thinking of our retired Pope Emeritus Benedict XVI, I must point out to you while he is resident in a former monastery, it does not make him a monk. He is living out a life of prayer, which some people associate with monasticism, but he has not sworn any monastic vows.