Have you considered using the DIM CLASS directive as in:
po$["po.order_items.1.item"]="widget1"
po$["po.order_items.1.price"]="10.25"
dim class po$["po.order_items.1.price"]="N"
po$["po.order_items.1.qty"]="2"
dim class po$["po.order_items.1.qty"]="N"
Or perhaps make a simple function of it:
DEF FN_poSetNum(local idx$, local val)
po$[idx$]=str(val)
dim class po$[idx$]="N"
return
End Def
Then
po$["po.order_items.1.item"]="widget1"
fn_poSetNum("po.order_items.1.price", 10.25)
fn_poSetNum("po.order_items.1.qty", 2)