The phrase "shall so" is grammatically correct and can be used in written English.
It is often used to emphasize or confirm a previous statement or action. Below is an example of how you could use it in a sentence: "I promise to deliver the project on time, and shall so do with the help of my team."
Their high foliage shall drop heavenliness upon me; my feet in contact with their mighty roots, immortal vigor shall so steal into me.
The letter confirms that Schmidt "will be in good standing with BCC as if the Incident never occurred, and BCC's records shall so reflect".
… On the grounds of motivation, academic records, potential promise, endorsement by persons capable of reasonable judgments, personal appearance and decorum, maturity, and probable contribution to balance in the class, I believe Mr. Bakke must be considered as a very desirable applicant and I shall so recommend him".
If the Attorney General concludes that the law cannot be enforced constitutionally, the Attorney General shall so notify the President, who will convey the Attorney General's determination to the committees of the House and Senate with jurisdiction over the relevant agency.
In 1651, the colony passed its prohibition on private purchases of Indian lands: [N]o purchase shall be made of any Land of the natives for a plantation without the consent of this State, except it bee for the clearinge of the Indians from some particular planatations already sett down upon; and if any shall so purchase, they shall forfeit the Land so purchased to the Collonie..
Should any officer of the United States believe that a law he or she is responsible for enforcing cannot be implemented in a constitutional fashion as written, that officer shall so inform the head of, and general counsel to, the relevant agency, who shall determine, in the first instance, whether the law is constitutionally enforceable.
