in 10.0 it was not possible ... as you said ... link type definition but not relationship constraints ...
need to create the XML manually
for example
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE NmLoader SYSTEM "standardX20.dtd">
<NmLoader>
<!-- ***************************************************** -->
<!-- ** Configurable Link Valid Relationships ** -->
<!-- ***************************************************** -->
<csvCreateDefaultAssociationConstraintItem handler="com.ptc.core.meta.type.mgmt.server.impl.association.LoadDefaultAssociationConstraintItem.createDefaultAssociationConstraintItem" >
<csvlinkType>com.richemont.MyLink</csvlinkType>
<csvroleAType>com.ptc.myWTpart</csvroleAType>
<csvroleBType>com.ptc.myObject</csvroleBType>
</csvCreateDefaultAssociationConstraintItem>
</NmLoader>