FCLoopRemoveNegativePropagatorPowers[exp] rewrites
propagators raised to integer powers as products.
SFAD[{q, m, -1}]
ex = FCLoopRemoveNegativePropagatorPowers[%](q^2-m+i \eta )
q^2-m
ex // StandardForm
(*-m + Pair[Momentum[q, D], Momentum[q, D]]*)SFAD[{q, m}, q + p, {q, m, -2}]
ex = FCLoopRemoveNegativePropagatorPowers[%]\frac{1}{(q^2-m+i \eta ).((p+q)^2+i \eta ).\frac{1}{(q^2-m+i \eta )^2}}
\frac{q^2-m}{((p+q)^2+i \eta )}
ex // StandardForm
(*FeynAmpDenominator[StandardPropagatorDenominator[Momentum[p, D] + Momentum[q, D], 0, 0, {1, 1}]] (-m + Pair[Momentum[q, D], Momentum[q, D]])*)