If I understand your question correctly, you seem bothered by the interaction between a source featuring a certain output impedance \$Z_{th}(s)\$ and loaded by a switching converter offering an input impedance \$Z_{in}(s)\$. A closed-loop-operated converter offers a negative incremental resistance (\$arg(Z_{in}(f))=-180°)\$ as long as the converter rejects the input voltage variations. This is observable only in the low-frequency portion of the spectrum, where the loop gain \$T(s)\$ is the highest. As you soon as you start increasing the frequency, the loop gain falls and the argument of \$Z_{in}\$ becomes less negative. It becomes that of the open-loop-operated converter beyond crossover. The source output impedance is often dictated by the EMI filter output impedance you insert to fight perturbations. The equivalent schematic once a filter is inserted looks like that:

If you determine the voltage driving the converter and rearrange the expression, you can see the presence of a minor loop characterized by the ratio of \$Z_{th}\$ by \$Z_{in}\$. If this ratio becomes 1 - otherwise speaking you have overlapping points when you plot both magnitudes on a common-scale Bode plot - then you have condition for oscillations if the argument of \$\frac{Z_{th}}{Z_{in}}\$ becomes \$-180°\$. The difficulty is to assess this argument and Bode can sometimes be misleading. In this case, apply the Nyquist criteria and you will always be safe. Second option, damp \$Z_{th}\$ so that no overlap can be found - \$|Z_{th}(f)|<<|Z_{in}(f)|\$ - and you don't care about the phase anymore! You can find more information regarding interactions between a filter and a switching converter in a seminar taught at APEC this year.