The right side of a "Variable Assignment" is considered quoted (no splitting or globing):
LESS=+'/A variable may be assigned' man bash
A variable may be assigned to by a statement of the form:
Word splitting is not performed, ... . Pathname expansion is not performed.
$ echo "$var"
To get an asterisk to expand, you need to use it un-quoted:
var="$(echo *)" ### Expand to the list of files on pwd.
In your case:
$ IMP="$( echo "firstname.lastname@example.org:/data/$ID/EER_DATA/"*"/ImpostorScores.txt")"
$ echo "$IMP"
That something will be expanded if the whole path match, that is, a tree like this actually exists:
$ tree email@example.com\:/
Note: I removed the second
$ID from your source, easy to re-place.