In mapinfo it is possible to save an output without creating new shp. Either by saving into the input- layer or to a temporary file. For example if you want to ”union” two layers you may want to get the output into one of the input layers. Anyone know of smiliar possibilities in qgis?
Typically in the OGR world (which QGIS uses) there is an append method. https://www.gdal.org/ogr2ogr.html
The flag does not appear to be exposed in the QGIS Toolbox modules.
There do appear to be some plugins that might do the trick https://plugins.qgis.org/plugins/tags/append/ in particular the "Append to Feature Layers" https://plugins.qgis.org/plugins/AppendFeaturesToLayer/