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Transmission system operators (TSOs) increasingly encounter small-signal instabilities (SSIs) when integrating converter-based resources into modern, converter-dominated power systems. Identifying and mitigating such instabilities is challenging due to complex converter interactions among multiple converters under a wide range of operating points (OPs). The challenge is particularly critical during the grid-connection process, where TSOs must validate stability before new converter-based resources are integrated into the system. Intellectual property rights (IPRs) further complicate the analysis by restricting TSOs' access to original equipment manufacturers (OEMs) and plant owners (POs) detailed models, which pose difficulty in coordinating studies and communicating analysis insights among multiple parties. Hence, methods that can address both the inherent complexity of SSIs and the IPR restrictions are needed. To address this challenge, this paper proposes a collaborative study framework between the TSO and OEMs/POs, which preserves OEM/PO IPRs while enabling an automated system-level smallsignal stability risk assessment and root cause identification across the OP space. The framework considers black-box OEM/PO state-space models, which are combined with the TSO's power system model using the component-connection method, enabling the eigenvalue-based stability analysis. The effectiveness of the study framework is demonstrated on a system with two gridfollowing wind-power plants (WPPs) and two parallel grid-forming high-voltage direct-current interconnections, where one WPP is undergoing grid-connection. Electromagnetic transient simulations validate the theoretical analysis of the framework.