typescript extend an interface as not required

If you want Success and Message to be optional, you can do that:

interface IAppVersion {
    OSVersionStatus: number;
    LatestVersion: string;
    Success?: boolean;
    Message?: string;
}

You can't use the extends keyword to bring in the ISuccessResponse interface, but then change the contract defined in that interface (that interface says that they are required).

A bit late, but Typescript 2.1 introduced the Partial<T> type which would allow what you're asking for:

interface ISuccessResponse {
    Success: boolean;
    Message: string;
}

interface IAppVersion extends Partial<ISuccessResponse> {
    OSVersionStatus: number;
    LatestVersion: string;
}

declare const version: IAppVersion;
version.Message // Type is string | undefined