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Adding an effects loop to a 5F2 (or its close relative, the 5F1) amp is generally considered not worthwhile or practical by the guitar community.

The primary reason is that the 5F1/5F2 circuit is a single-channel, low-gain amplifier where distortion is produced primarily in the power section, not the preamp. An effects loop is designed to place modulation and time-based effects (like delay and reverb) after preamp distortion to keep them clear. Since the 5F1 lacks preamp distortion, placing effects in a loop offers little sonic benefit over running them into the front of the amp.

Key technical constraints include:

Signal Levels: The signal point between the preamp and power amp in these vintage circuits is often too hot (high amplitude) for standard pedal inputs, potentially causing clipping or noise unless significant buffering or attenuation is added.
Circuit Simplicity: The amp has only one gain stage. Modifying it to insert a loop requires complex changes to the bias or gain structure, which can degrade the iconic "champ" tone.
Alternative Solutions: If you need effects after gain, it is more effective to use the amp clean with pedals in front, or use a re-amping device (like a Fryette Power Station or Boss Waza TAE) to send a line-out signal through effects and back into the amp's power section.
While passive FX loop mods exist for similar circuits like the 5E3 (which has two gain stages), they are rarely recommended for the 5F1/5F2 due to the lack of preamp saturation to justify the modification