No, I can not do injection in DOM with Angular tools. It's bad idea do it and it is not possible now. AgGrid doesn't support customization with tools like ngx-scrollbar which using ng-content for wrapping components and other elements. May be better way using other tools which working by another way or using webkit customization which supports not all web browsers.
UPDATE
Also I try to use smooth-scrollbar. I managed to get better result. I bind him to element .ag-body-viewport. It works fine but it scrolling only content of grid without header be careful. smooth-scroll bar doesn't have options for horizontal and vertical scrollbar as a different units. I know how issue can be solve. Vertical scrollbar must be bind to .ag-body-viewport and horizaontal scrollbar must be bind to .ag-root. If your can find scrollbar which let you do it that you can solve this problem. May be I write special component for Angular in near future and then add link to this answer. If you do it faster you can add yourself link or you can add link to already existing packages.