I don't know about the software, but this is an algebra problem.
Take the % x Total x utilization x AA% for each addition. When you add all those together, they equal the total IBU. Then you solve for "Total". You'll need a utilization chart based on boil minutes and gravity.
Alternatively, if you don't like algebra and charts..... plug in 1 oz for the bittering addition and scale the rest based on the percentages. eg if the bittering addition is 23% and the 15 minute addition is 40%, 1 oz bittering means 1 x 40%/23% = 1.74 oz at 15 minutes. Plug all those in to your program to calculate IBU. Whatever % your IBU is off, multiple all the additions by that amount. eg. if you are 10% low, the 1 oz bittering becomes 1.1 oz and the 1.74 @ 15 minutes become 1.91 oz.
BTW - what recipe are you trying?