No, unfortunately, you cannot. Only the full-length slot includes the SATA lane that is necessary for a mSATA SSD to be added.
What you might be able to do (though I rather doubt if there is really enough room inside the "K" chassis to attempt this) is purchase a SATA to mSATA board, attach your second mSATA SSD to this board and then connect it to the additional SATA port on the D54250WY board. Another option might be to contact Intel Customer Support and find out if you can get a bare "H" chassis and move your board into it).
Frankly, this isn't really worth it. Your best bet is to get a good external USB 3.0 HDD and use it. For even better performance (though likely more expensive), get a external USB 3.0 to SATA chassis and put a SSHD into it.
I thought that was the case.
Thanks for the advice! I'll take a look at the options you mentioned