You can have a composite key with a number and varchar as this is usual we have such in my project something like CODE and ID , where code is varchar and ID is number.
What do you mean by impact or such kind of scenario?
I you are looking to combine the values into a single column you could ORA_HASH() the character column before concatenation ... then you have a single value numeric for indexing and performance.
Key = TO_NUMBER(ORA_HASH(char column) || numeric column)
Since you won't be using the hash for statistical uses, you don't need the optional parameters.