If you have a "pivot motif", which shares, for example, rhythmic properties of theme A and theme B, and is used to move between the two themes, could you consider this a modulation? What other musical phenomena can be described as a modulation?
Between two motives, the term often used is thematic transformation. This is especially common in the music of Wagner, who often creates new Leitmotive out of (or with heavy reference to) prior motives.
As for modulation, the notion of metric modulation became relatively common in the twentieth century. (Others called it tempo modulation.) In a metric modulation, composers change the meter and/or subdivision of that meter, and based on whether the beat and/or the subdivision stays constant, the tempo changes.
Consider the following example:
We begin at quarter = 120. In m. 2, the beat stays constant, so we're still at 120bpm, it's just that it's the dotted quarter getting the beat. But in m. 4, it's the eighth notes that stay constant, which means we return to 4/4 time not at the prior 120bpm, but at 180. Thus we've "modulated" the meter (or more accurately, the tempo) by using the eighth notes as a type of "pivot articulation." (This isn't a common term, but I think the metaphor is helpful.)
You can find metric modulation in music by Elliot Carter, Milton Babbitt, and others.
If 'modulation' didn't already have an accepted 'music theory' meaning, it might be a good term for what you describe. But it does, so I think using it that way would be merely confusing. Find another word to describe it.
'Modulation' is used in synthesis to describe one waveform affecting another. 'Frequency modulation' and 'Amplitude modulation' are both used (as in radio). Far enough away from 'music theory' to avoid confusion I think.
I would call your "pivot motif" a thematic transition, perhaps? Depending on how long you are using that pivot motif, it might fall into the category of being a development of theme A.