Transfer Logins Task, Can't Set LoginsList Or DatabasesList From Variable

I need to make a general package to copy SQL databases between servers.
A package with fixed task properties work fine.
But my plan is to make a general package and use varables to handle parameters like connection strings, server names, database names etc.
My problem is the Transfer Logins Task and its collection properties like LoginsList and DatabasesList which uses string collections.
In my understanding I can't make a ordinary variable in "design mode" for string collection. For that reason I wrote some code in a Script Task to set an Object variable to a string collection. And then use the object variable to set the LoginsList or DatabasesList property.

But I get following error:
TITLE: Microsoft Visual Studio
------------------------------
Nonfatal errors occurred while saving the package:
Error at Transfer Logins Task: The data type of variable "User:ourceDB_list_o" is not supported in an expression.
Error at Transfer Logins Task: Reading the variable "User:ourceDB_list_o" failed with error code 0xC00470D0.
Error at Transfer Logins Task: The expression "@[User:ourceDB_list_o]" on property "DatabasesList" cannot be evaluated. Modify the expression to be valid.


My code to set the object variable:

Dim DBs As System.Collections.Specialized.StringCollection = New System.Collections.Specialized.StringCollection()

DBs.Add(Dts.Variables(0).Value.ToString())

Dts.ExecutionValue = DBs


Grateful for any guidance.

Regards, Jonas