Scan architecture with compression support has been a de-facto solution in recent designs. It has been shown that compression levels and the quality attained highly depend on the ratio of encodable test patterns. We observe that whether a test pattern is encodable by a decompressor is a direct consequence of whether its care bit distribution is compatible with the correlation induced by the decompressor. Therefore, we present a series of "add-on" blocks that can be utilized to control the care bit distribution and thus to improve the encodability of stimulus decompressors. We show that compression levels attained by fan-out and XOR decompressors can be significantly enhanced via the proposed add-on blocks and the supporting algorithms.