<assign>
<copy>
<from variable="getApproverDetails_OutputVariable"
part="UserData"
query="/ns20:UserData/ns20:CustomAttribute[@Name='YOUR_UDF']/ns18:Value"/>
<to variable="YOUR_VAR"/>
</copy>
</assign>
Example:
<assign>
<copy>
<from variable="getApproverDetails_OutputVariable"
part="UserData"
query="/ns20:UserData/ns20:CustomAttribute[@Name='Email']/ns18:Value"/>
<to variable="approverEmailID"/>
</copy>
</assign>