If your focus is to keep out moisture, don't use the tire pump. The act of compressing air increases the density of the air but, since water is not compressible, also increases the percentage of water in the air. Most tire compressors do not have a drier element so you may just be increasing the humidity.
Since it is a spare, why not just take it in for an internal inspection and then ask them to only fill it to 500 or less.
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