Hi Teo,
As Iion is a protected word in EasyML. A possible solution could be adding a parameter that would modify Iion. For example, you can create a parameter called factorcurrent=1;.param(); and when you have Iion, you can use the created variable to multiply Iion (Iion = factor current*(INa+...).
I hope this can help in your simulations.
Best,
Jorge