In my agency, a large single mission federal one, we hire all of our forensic people specifically for those positions and it is a career ladder completely separate and different from our 1811's (criminal investigators/special agents).
We have several regional laboratories, Washington DC, Miami, Dallas, for example and when we need some specialized forensic examination done in the field, the labs fly or drive someone out. An example would be if we want a warehouse ion scanned to be tested for the former presense of any drugs.
But, there is no cross over. If a lab technition, or an agent, want to switch jobs, they have to apply just like anyone in the general public.
"Man who say it cannot be done should not interrupt man doing it."
Old Chinese Proverb