You are in the right direction, just missing one intermediate step of importing property into impex macro key using correct processor.
1 - define your key in project or local properties file
mykey=myvalue
2 - Import above in impex macro using ConfigPropertyImportProcesser
GenericItem[processor=de.hybris.platform.commerceservices.impex.impl.ConfigPropertyImportProcessor];pk[unique=true]
$config-key=$mykey
3 - Run your impex with imported config macro
INSERT_UPDATE User; uid[unique=true]; specialKey
;testuser;$config-key;
NOTE : Moving key=value
from project.properties to local.properties will not make a difference. The later file only has precedence over the values defined in project.properties and helps you to define/maintain/override such pairs as per application/deployment environment needs.