Complex sentence: The Malaspina Expedition of 1791 also brought Filipino seamen to Alaska, after deserters in the originally Spanish crew had been replaced by conscripted Filipino sailors when Spanish corvettes had stopped in Manila.
Split into simpler sentences: The Malaspina Expedition of 1791 also brought Filipino seamen to Alaska. They were replacements for the deserters from the originally Spanish crew and had been conscripted when Spanish corvettes had stopped in Manila.