I was messing around with this a little and it looks like the only dice string that accepts modifiers as additional dice is /rollunder. /pbta results in a numerical modifier.
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Thats right. There is a single modifier stack so only one set of modifiers can be stacked. /pbta only uses numeric modifiers (like the majority of game systems). I will upload a /rollover later this week that works exactly the same way as /rollunder eg 5d6x5 would roll 5d6 and count 5s and 6s. It will accept modifiers that increase and decrease the dice count.
You are welcome to check out the dice rolling scripts and create your own. If you do so I will roll them into MoreCore and credit you for them and more people can use them.