The only way to get the skills onto a call is with the Transfer to ACD action. You could use the Update Data action to set skill variables that the transfer action would then utilize.
Something like this, in the update data set one or more skill variables.
Then in the transfer action, you can add one or more skill fields and then switch the fields to expression where you can specify the variables you set in the update data.
Unfortunately you couldn't use a collection variable because the transfer action is expecting a specific known number of skills at design time. You could set the expression to something like this: GetAt(Task.skillCollection, 0), but that still requires you to know exactly how many skills you will be setting.