You did not provide what you have setup , notably you could use couple approaches to accomplish your requirement.
Notably if you Google [FDMEE multidimensional mapping] you will get a few hits which cover your use case.
Good luck and let us know how you make out.
As per admin guide -check if below helps.Its actually about precedence within a mapping type.You may try to segregate Explicit maps for your requirement from the Multi dimension maps.
When processing the source values for transformations, multiple mappings may apply to a specific source value. The order of precedence is Explicit, Between, In, Multi-Dimension, and Like. Within Between and Like types, mappings can overlap.
The rule name determines precedence within a mapping type. Rules are processed in alphabetical order of the rule name within a mapping type. Numbers may also be used to help with ordering. For example, if numbering by tens or one hundreds, you can insert new rules between existing ones. So if rules are numbered 10,20,30, you can add a rule that starts with 25 and need not rename other rules.
Simple answer is you cannot change the precedence definition for the mapping types i.e. Explicit maps will always be executed first. If you want an account to ve considered in combination with an ICP value in a multidimensional map then do not create an entry for it in the Explicit maps. You would then create the account/ICP multidimensional map for the specific combination of account/ICP and then a single Like mapping to capture all other instances of that account not captured by the multidimensional mapping.
I guess you could change your import format and concatenate account with ICP and use that as an explicit map.
Or you could use SQL on the 'In' map to do something clever like lookup the account and if the ICP is something...change the target.
Try looking at the script editor on the setup tab - you can run scripts after the mappings are done.
I have one running on AftProcMap which does a similar sort of thing on the TDATASEG table after mappings are completed.