It sounds like you are talking about something similar to pan sharpening, only instead of having a higher resolution panchromatic image, you have a higher resolution multispectral image. I assume that your Sentinel data is Sentinel-2a, and that you'd like your final image to end up with the same bands as the input Sentinel-2 data. Is that right? I'm afraid that ENVI does not have a tool to do this type of fusion. Also, theoretically, it is not possible, since your high resolution imagery only covers part of the wavelength range of the Sentinel-2a data. There is no information in the high resolution image that can be used to simulate bands outside of the red, green, blue wavelength region. So there is no way to get the information at those wavelengths to appear to have higher spatial resolution. I'm sorry that I don't have better news for you. Peg