The consonant [b] can be prevoiced, so it would seem, at least at first blush, that prolonging the closure for this plosive would entail prenasalizing it. I've tried to produce it without prenasalization only to find my cheeks inflating. But for all I know, I'm looking at this in a wrong-headed way. Thoughts?
First, you need to set a standard for "prolonging". If /b/ is some language typically has a duration of 100 msc in a given context (plus or minus 15) then extending the stop to 150 msc counts as prolonging. Second, you need to be dealing with a language (like French) where /b/ actually is voiced throughout, and if the goal is to produce prolonged voiced stops, you need to be fluent enough that you have acquired the phonetic tricks for maintaining voicing. Third, there are many such tricks: buccal and pharyngeal expansion, laryngeal lowering, nasal leakage, also "continuant closure" where closure of the stop is only partial at the left -- you don't notice it because the airflow is negligible though not zero. Nasal leakage is probably the historical underpinning of the historical sound change in some languages where voiced stops become "prenasalized". I suggest emulating voiced geminate (bilabial) stops under the tutelage of a speaker of Arabic (for example), if you want to overcome the tendency to insert a nasal at the beginning of the closure. This is actually a rather common coping strategy for producing consonants with challenging airflow (clicks for example: student often nasalize everything surrounding the click when producing Zulu words with clicks).
The phonological difference is simply that in [mb] you have two segments at the same place of articulation the first of which is a nasal sonorant and the second of which is oral. The phonetic difference is basically the same, only expressed in terms of articulations ("lowering the velum", "maintaining the labial closure"). Many languages that have geminate voiced stops also allow homorganic nasal plus consonant sequences, for example Luganda.