To add (something) to what has already been said; to make an additional remark or add a supplementary comment (polite).
一点申し添えるとすれば、納期は多少前後する可能性があります。
If I may add one point, the delivery date might shift somewhat.
先ほどの説明に申し添えると、費用には税が含まれます。
To add to the earlier explanation, taxes are included in the cost.
申し添えるならば、この条件は交渉の余地があります。
If I may add, this condition is negotiable.