Is it possible for nasals, fricatives, etc to be implosive?
Implosives other than stops (including affricated stops) are not attested, and there is theoretical reason to believe that they could not exist. In order to create negative oral pressure by downward movement of the larynx, the vocal tract must be sealed. Only stops have complete vocal tract closure. Downward movement of the larynx in the case of a nasal would have negligible acoustic result since air pressure would be quickly equalized through the nasal passages. In the case of fricatives, there might be some attenuation of airflow across the glottis to the point of constriction, but no vacuum.
Phonological theory lumps ejectives, implosives and laryngealized sounds together with a single feature, [constricted glottis], and the specific realization of the sound is a result of phonetic implementation, where, in essence, downward movement of the larynx as typifies an implosive is what is necessary to have both voicing and glottal construction, when there is no supraglottal escape for air.