Calculate simple arithmetic string in Scala
You might want to spend some time becoming more familiar with the Standard Library. You're doing a few things that are already provided for.

implicit class Converter(char: Char) {
  def toNum: Int = char - '0'
}


To convert a digit character into the value it represents: char.asDigit. If you have one or more digit characters in a string: digits.toInt

Your solution also fails for expressions like "2*2+3". It looks like the evaluateExpression() code doesn't anticipate that, in the recursive call evaluateExpression(pendingExpression), the value of pendingExpression might be a single digit.

But mostly, with the input under so many tight restrictions, you should consider what shortcuts might be available to you.

def evaluateExpression(s :String) :Int =
  s.split("\\+")
   .map(_.split("\\*")
         .map(_.toInt)
         .product)
   .sum