Many, many years ago, Heathkit had an FM generator with vacuum tubes that included stereo encoding.
There are boatanchors from the time of the introduction of stereo multiplex, and expensive units (maybe no longer available) from the solid-state era.
When I wanted to generate a suitable FM-stereo signal, I bought an interesting circuit board from a company that sells equipment to low-power FM broadcasters, and connected it into the external modulation signal input of a good RF generator with FM capability.
The board was the "Veronica" from AAREFF: I don't know if it is still available.
https://www.aareff.com/en/fm-transmitter-stereo-coder-kit/Its output is the equivalent of the output from an FM demodulator inside an FM receiver.
The board comes pre-wired (with cheaper components than I like), but requires an external +/- 15 V DC supply.
You will need to take care about the signal levels with respect to the "external modulation" input levels of whichever RF generator you use.
The "left" and "right" sides result from applying an audio signal (e.g., 1 kHz) to input "A" or input "B" of the "Veronica" board, to generate a "baseband" signal suitable for frequency-modulating the RF generator's carrier.
The generator is one of my boatanchors: Wavetek 2405, a synthesized 500 MHz generator with AM and FM modulation capability from eBay that needed re-calibration.
(Side comment: please use correct spellings of "MHz", etc. The "mHz" does exist, but is far from what you intend to say.)