The International Celestial Reference System (ICRS) is realized in the radio band by a catalog of radio source positions obtained by very long baseline interferometry (VLBI) measurements and called the International Celestial Reference Frame (ICRF). The current ICRF approved by the International Astronomical Union (IAU) at its XXXth general assembly in August 2018 is the ICRF3. Discussions are currently underway on the preparation of the next ICRF release, the ICRF4. In this paper, we investigate a possibility to improve the S/X version of the ICRF3 by constructing a combined catalog of radio source positions based in individual catalogs provided by VLBI analysis centers. This study is based on the analysis of the systematic errors of six most recent VLBI radio source catalogs using Vector Spherical Harmonics (VSH). In a first step, we determined the systematic differences between each input catalog and the S/X version of the ICRF3. These systematics were then applied to the input catalogs to transform them into the ICRF3 system. The average of these transformed catalogs is the combined catalog in the ICRF3 system (COMB1X). Applying the averaged systematics to the COMB1X catalog, we obtain the combined catalog in an independent system (COMB2X). The intercomparison of partial combinations allowed us to rescale the formal errors to make them more realistic. The two combined catalogs of radio source positions contain a few more sources than individual catalogs. The COMB1X catalog can be considered as an improvement of ICRF3 in terms of stochastic errors. The COMB2X catalog can be considered as an improvement of ICRF3 in terms of both stochastic and systematic errors. The COMB2X catalog reveals systematic error of a few tens of microarcseconds with respect to the ICRF3.