1. AFAIK it's not possible. You can add more upgrades in the XML, but it won't work.
2. BASE_UNIT_UPGRADE_COST and UNIT_UPGRADE_COST_PER_PRODUCTION (in GlobalDefines.xml)
(Also UNIT_UPGRADE_COST_MULTIPLIER_PER_ERA*, UNIT_UPGRADE_COST_EXPONENT*, UNIT_UPGRADE_COST_VISIBLE_DIVISOR, UNIT_UPGRADE_COST_DISCOUNT_MAX)
* I don't know if/how these 2 defines work.
For adjusting upgrade cost of individual units, you can try using a promotion with UpgradeDiscount, but I haven't checked if it works.