Referring to the SixPack Plus manual, section 2.1.1, it states: "The SixPackPlus card must be properly configured to indicate the amount of memory installed below it, or in other words, what its starting address should be. ..." Switches 1, 2 and 3 address this configuration; the highest starting address is 512K (1, 2 and 3 all = ON). The manual further states "Notice that the SixPackPlus automatically begins limiting the amount of usable memory on the card when its starting address is configured at 320k or higher. The sixPackPlus can be addressed only within the first 640k of the PC, and its starting address can be no higher than 512k."
From this one might conclude that no, you can not use the memory on board the SPP for anything other than conventional memory, and only that portion of conventional memory that your PC has not already populated.
In other words: No free lunch.
-CH-