Recursively import all nested references in a Maya scene.

importAllRefs.mel

// IMPORT ONLY TOP LEVEL REFERENCES
string $references[] = `ls -type "reference"`;
for ( $r in $references)
{
    string $ref = `referenceQuery -filename $r`;
    file -importReference $ref;
}

// RECURSIVELY IMPORT ALL REFERENCES
{
string $topLevelReferences[] = `file -q -r`;
do {
    string $topLevelReferences[] = `file -q -r`;
    for ($ref in $topLevelReferences){
        file -importReference $ref;
    }
}
while (size(`file -q -r`) > 0);
}