You can add modules, there are multiple ways to do it. Gadgeteer interface (daisy chain), something like a three pin eblock, SPI, I2C, GPIO etc.
It all depends on your harware knowledge and ability. Alternatively you just need to watch out for these sensors to be made available by GHI or one of the others in the Gadgeteer ecosystem.
I just happened to mention a few sensors that might actually be added by GHI, but I’m meaning if I wanted to add modules that wouldn’t normally be made for it.
Like a connection to an FPGA, or sending output to a dedicated HDMI board, things normally used to extend an FPGA board.