Unfortunately you cannot define a member var + function with same name. i.e. the following is a compiler error:
interface foo {
a();
a:number;
}
Meanwhile you can do :
(<any>jQuery()).modal.Constructor.prototype.enforceFocus = function () {};
If TypeScript had type unions, which it doesn't (you can vote for these here https://typescript.codeplex.com/workitem/1364) you could potentially do:
interface foo {
a:()=>void | number;
}