is there anyway to add existing item as Matrix subitem ?

Yes, set the Parent Item on that item at "SubItem Of" body level field.

but even so, in the parent item, that new item will not be shown under Accounting -> Matrix Items subtab, right ?

Yeah, it won't. in order to display child item on Parent item, you need to set it via UE script. Adding the script on the save of child item, and set that item on Parent item either on any custom multi-select field or you can have custom record link to that.

so you're saying that we don't use the default matrix item link, but using custom link between parent and children items, right ?