Ambisonics, a sound field reproduction technique, can present spatial audio with high accuracy. However, it has not been widely adopted due to the hardware requirements it imposes. In consequence, very few Ambisonics encoded contents are available. End-users will find it more attractive to invest in Ambisonics systems if they can be made backwards compatible with existing recordings. In this paper, a technique to spatially extrapolate stereo sources for listening using Ambisonics reproduction systems is introduced. The proposed approach makes use of the spatial information already encoded in most modern stereo recordings. This spatial information is usually used to decode multichannel audio signals, such as 5.1 channel surround. Unlike these technologies, the proposal attempts to extract a continuous description of the 360-degrees horizontal sound field which would result in the original stereo recording. The output is encoded using the circular harmonic functions, making it adequate for reproduction using horizontally-distributed loudspeaker arrays.