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In the strict sense, one cannot add RAM using a PC card.
In the general sense of 'accessible file storage space', one can.
If there were a PC Card hard disk, one could perhaps add file storage space that way. (I am not sure there is such a thing.)
There are PC Card adapters for memory cards: for instance, for CF cards.
And there are PC Cards which themselves are storage devices with memory space:
Supposedly one can add more working memory for Windows using a USB flash drive. When I have seen this tried, though, the improvement was not noticeable, simply because the access speed for such 'memory' is much slower.