You can first try if a compressor with ≈ 400 ms attack, 30 ms decay, and rather violent threshold/ratio (something like -40 dB and ≥8:1) settings does the trick. If not, try a more conventional compressor setting (if the synthesized guitar has little dynamic range, that may not even be necessary) and put an expander/noise gate after it, which triggers only the note plucks. Fast attack and ≈ 400 ms decay.
If neither of these works satisfyingly, you will need to resort to specialized envelope tools, or to using a proper palm mute sound in the first place.
Note that, for a distorted sound, all tweaks can only work properly if you apply them before the distortion. Once the signal has passed the amp, it will be alnost impossible to do anything about the playing technique characteristics.