Is there a standard format for compact 56-bit DES keys?
The one time I've seen a system compress a DES key into 7 bytes, it did it by logically placing the 8 bytes of the standard DES key into an 8x8 table, and then flipping the table along the diagonal axis, so that bit x of byte y of the standard DES key was mapped to bit 7-y of byte 7-x of the compressed key. For example, the first byte of the compressed key consisted of the msbits of all 8 bytes, and the second byte of the compressed key consisted of bit 6 of all 8 bytes, etc.  This moved all the lsbits of each byte into the last byte, which was discarded.

However, I don't believe this was, in any way, standard; it just happened to be what the system I saw did.  This particular transform does have the advantage that the same routine can convert both directions between the standard DES format and the compressed format (that has all the ignored bits in the last byte).