typescript extend an interface as not 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