Well, hard to describe it without actually writing a grammar, but here's an action plan
You need to implement "for" operator:
forExpr.Rule = Expr + "for" + Number;
You also need to define an "sequence expression" - when several expressions are delimited by space:
seqExpr.Rule = MakePlusRule(seqExpr, Symbol(" "), Expr);
Test the grammar in parsing - once it parses correctly your samples, you need to implement "execution" part.
You need to define custom AST nodes for both non-terminals (forExpr and seqExpr), and implement Execute method in them accordingly.
So forExprNode.Execute should return string composed of n copies of the first arg; seqExprNode.Execute should concatenate all child expressions with space in between and return the resulting string.