The phrase "adding still further" is correct and usable in written English.
It can be used when you want to emphasize that you are increasing or extending something beyond what has already been mentioned.
Example: "The project has faced numerous challenges, and adding still further complications, the deadline has been moved up."
Alternatives: "furthermore" or "in addition".
And on rehearing the Court of Appeals added still further error.
Such a result will add still further to the confusion and chaos of judicial rate making.
An acute shortage of food added still further to his problems.
In 1669 the King added still further to Colbert's dignities by making him responsible for the intellectual and artistic life of the country, as secretary of state for the king's household.
This skywriting adds still further to the romantic reputation of the young officer, so that when he announces that he is holding a photographic exhibition in his apartment, influential figures from the armed forces and Chilean high society show up for the opening.
All of this adds still further to a personal legacy that as long ago as 2000 was sufficient to have Warne named by Wisden Cricketers' Almanack, the game's venerable annual record, as one of its Five Cricketers of the Twentieth Century.
Or was it deemed right to add still further that they should not be restrained to the powers already named --besides the powers already menamed --besidespowers may be assumed hereafter as conamed --besidesicathen in this constitution.
Propagation and development of these standards, together with training in basic informatics for pathologists, would add still further to the efficiency and usefulness of the approach.
The risk, though, is that the threat of Islamic militancy helps to defocus the alliance further, adding still more tasks to its "to do" list, reducing consensus about where the greatest danger lies and the costs involved in addressing it.
